Various metallic materials capable of exhibiting shape-memory characteristics are well known in the art. These shape-memory capabilities occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. In particular, it was discovered that alloys of nickel and titanium exhibited these remarkable properties of being able to undergo energetic crystalline phase changes at ambient temperatures, thus giving them a shape-memory. These shape-memory alloy (xe2x80x9cSMAxe2x80x9d) materials, if plastically deformed while cool, will revert, exerting considerable force, to their original, undeformed shape when warmed. These energetic phase transformation properties render articles made from these alloys highly useful in a variety of applications. An article made of an alloy having shape-memory properties can be deformed at a low temperature from its original configuration, but the article xe2x80x9cremembersxe2x80x9d its original shape, and returns to that shape when heated.
For example, in nickel-titanium alloys possessing shape-memory characteristics, the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is often referred to as a thermoelastic martensitic transformation. The reversible transformation of the NiTi alloy between the austenite to the martensite phases occurs over two different temperature ranges which are characteristic of the specific alloy. As the alloy cools, it reaches a temperature (Ms) at which the martensite phase starts to form, and finishes the transformation at a still lower temperature (Mf). Upon reheating, it reaches a temperature (As) at which austenite begins to reform and then a temperature (Af) at which the change back to austenite is complete. In the martensitic state, the alloy can be easily deformed. When sufficient heat is applied to the deformed alloy, it reverts back to the austenitic state, and returns to its original configuration.
SMA materials previously have been produced in bulk form, in the shape of wires, rods, and plates, for utilities such as pipe couplings, electrical connectors, switches, and actuators, and the like. Actuators previously have been developed, incorporating shape-memory alloys or materials, which operate on the principal of deforming the shape-memory alloy while it is below its phase transformation temperature range and then heating it to above its transformation temperature range to recover all or part of the deformation, and, in the process of doing so, create moments of one or more mechanical elements. These actuators utilize one or more shape-memory elements produced in bulk form, and, therefore are limited in size and usefulness.
The unique properties of SMA""s further have been adapted to microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) applications such as micro-valves and micro-actuators by means of thin film technology. Micro-actuators are desirable for such utilities as opening and closing valves, activating switches, and generally providing motion for micro-mechanical devices. The most well-known and most readily available SMA is an alloy of nickel and titanium. NiTi SMA has been extensively investigated as one of the most promising materials for MEMS such as microvalves and microactuators. NiTi SMA features the major advantages of having a large output force per unit volume, and the capability to serve as structural components as well as active components. It is reported that the advantageous performance of micro-actuators is attributed to the fact that the shape-memory effect of the stress and strain can produce substantial work per unit of volume. For example, the work output of nickel-titanium shape-memory alloy is of the order of 1 joule per gram per cycle. A shape-memory film micro-actuator measuring one square millimeter and ten microns thick is estimated to exert about 64 microjoules of work per cycle. With a temperature change of as little as about 10xc2x0 C., this alloy can exert a force of as much as 415 MPa when applied against a resistance to changing its shape from its deformation state.
Previous processes involving MEMS have involved two fabrication techniques: machining of bulk SMA sheets or wires and deposition of SMA films from a NiTi alloy target. Unfortunately, these processes feature miniaturization and productivity limitations. Machining and assembly for MEMS devices using bulk SMA materials restricts manufacturing object size such as thickness of sheet and diameter of wire. Deposition processes have the potential of miniaturization and mass production. Deposition of a sputtered flux from a multicomponent target has been practiced for the growth of thin alloy films of a desired composition. It is known, however, that the composition of the sputtered flux varies with the polar angle of ejection from the target because of the different angular distributions of the individual target constituents. Conventional sputter deposition systems typically include a vacuum enclosure forming a sputter deposition chamber in which a circular sputter target is mounted facing opposite a wafer substrate surface. A magnetron cathode source, with means for producing a magnetic field, is set behind the circular target material. After the sputter deposition chamber has been pumped out to create a vacuum therein to the order of 10xe2x88x925 Pa, a sputter process gas, such as argon, is fed into the chamber and held at a fixed process pressure. The magnetron cathode generally features a center magnet and an annular magnet surrounding the outer circumferential edge of the center magnet, which develops a magnetic field across the circular target. When voltage is applied, a discharge occurs between the target and the substrate and the target material undergoes a sputtering action and a film made of the target material is deposited on the substrate. To acquire shape memory properties, the deposited film material must be deposited on a heated substrate, or a post-deposition annealing step must be applied to the film.
Although numerous potential applications for shape-memory alloys now require materials featuring phase transformation temperatures above about 100xc2x0 C., the martensite start point for the common commercially available nickel-titanium alloys barely exceeds about 80xc2x0 C. In order to meet higher temperature applications, ternary alloys have been investigated, using various additional metallic elements. For example, substitution of noble metals (Au, Pd, Pt) for Ni in NiTi alloys successfully accomplishes higher temperature phase transformations, but the costs introduced are somewhat prohibitive for many commercial applications. Ternary nickel-titanium shape-memory alloys including a zirconium or hafnium component appear to be potentially economical high temperature shape-memory candidates.
Devices, such as actuators, made from shape-memory thin film with high transformation temperatures offer two advantages over Nixe2x80x94Ti SMA film that is currently available: they can be used at higher ambient temperature where Nixe2x80x94Ti film would not be functional; and, they can be operated at higher frequency due to faster response time that is primarily limited by rate of heat dissipation to surroundings. Although there have been attempts to develop high-temperature SMA thin films particularly with Nixe2x80x94Tixe2x80x94Hf composition, the results were unsatisfactory, because the increase of transformation temperatures by additional hafnium was inadequate, and, the mechanical properties were unsatisfactory for any real applications.
Accordingly, there exists a challenge to develop a reliable process for producing microns-thick thin films of high temperature shape-memory alloys.
A magnetron sputtering deposition process now has been developed for forming a thin film deposit of a Nixe2x80x94Tixe2x80x94Hf ternary shape memory alloy on a substrate by having high transformation temperatures and good shape-memory and mechanical properties. The disclosed method of forming a thin film deposit of a ternary shape memory alloy on a substrate by sputtering deposition comprises arranging a Nixe2x80x94Tixe2x80x94Hf target and a substrate within a deposition chamber; maintaining a working distance from the target to the substrate of about 83 mm to 95 mm; heating the substrate to a temperature high enough to induce in-situ crystallization; introducing a krypton working gas into the deposition chamber; applying appropriate level of deposition power so the deposition rate is about from about 6 xc3x85 per second to 120 xc3x85 per second; and, depositing a Nixe2x80x94Tixe2x80x94Hf shape memory alloy film having a composition ranging from Ni48(TiHf)52 to Ni50(TiHf)50.
Pursuant to the presently disclosed method, planar magnetron sputtering is used to conduct thin film deposition, employing a target composition selected to produce a deposited alloy film having a composition ranging between Ni48(TiHf)52 and Ni50(TiHf)50. It has been determined that the composition of the thin film is an important factor in determining the transformational and mechanical characteristics of the SMA thin film. It was found that maintaining the thin film with 48 to 50 at.% of nickel (with balance of titanium+hafnium), preferably with nickel 48.5 at % to 49.5 at %, serves to ensure the thin film has high transformation temperatures as well as good mechanical strength. Particularly preferred are Nixe2x80x94Tixe2x80x94Hf alloy films wherein the Hf component ranges between about 10 at % and about 30 at %, preferably between about 15 at % to 25 at %.
It further has been determined that proper selection of the sputtering parameters, including substrate temperature, working gas composition and pressure, substrate-to-target distance (xe2x80x9cworking distancexe2x80x9d), deposition power, and sputtering rate, also is important to control the mechanical characteristics of the thin film.
The substrate should be heated to a temperature high enough to induce in-situ crystallization. It has been determined that both the temperature of the substrate during deposition, as well as the procedure of heating the substrate, have significant impact upon properties of the as-deposited alloy thin films. Crystalline films that exhibit favorable shape memory effect can be obtained when the substrate temperature reached between about 350xc2x0 C. and about 450xc2x0 C. during deposition. To heat the substrate to this deposition temperature, it is preferred to follow a routine comprising first to heat the substrate to a temperature higher than deposition temperature, preferably between about 400xc2x0 C. and about 450xc2x0 C., at the commencement of deposition, and then to allow the substrate to lower in temperature gradually, and preferably down by about 50xc2x0 C., to about 350xc2x0 C. to about 400xc2x0 C., during the deposition process. This pre-heating procedure serves to produce film deposits with improved roughness and shape memory properties.
Utilizing krypton as working gas instead of conventionally used argon is preferred for improved thin film properties. The krypton gas pressure during deposition preferably is about 0.5 mTorr and about 3.0 mTorr. A pressure ranging between 0.5 mTorr and about 1.5 mTorr is particularly preferred.
The presently described method further involves proper setting of deposition power and working distance in order to ensure good thin film quality and uniformity while maintain a reasonable deposition rate. Depending on the target size, deposition power should be adjusted in order to realize deposition between about 6 xc3x85/sec to about 120 xc3x85/sec. In selecting the working distance, one has to consider the focus of plasma that affects compositional uniformity of the deposit. It has been found that by setting working distance at from about 83 mm to about 95 mm, material within 5xe2x80x3-diameter area is useful with respect to 8xe2x80x3-diameter target.